The electricity infrastructure allows transferring electricity from supply sources (generation or electricity storage) to distribution systems and ultimately to the end users. The system currently undergoes considerable change with the strong growth in fluctuating renewable energy generation such as wind and photovoltaic energy. Furthermore, continuing load growth and increasing regional power transfer in interconnected networks lead to a stressed and less secure power system operation in general.
Large-scale electricity storage systems coupled to the power generation may become vital to compensate for the fluctuation of power output in connection with such renewable energies and to deal with the additional requirements imposed on the system. Thus, electricity storage systems are to become an important element of the electricity infrastructure of the future.
There are various types of electricity storage systems such as electrochemical batteries, flow batteries, capacitors, compressed air energy storage (CAES), Flywheel Energy Storage, Pumped Hydroelectric, Superconducting Magnetic Energy Storage (SMES) or Thermal Energy Storage. Familiar battery technologies include lead-acid, nickel-cadmium (NiCad), lithium-ion (Li-Ion), natrium/sulfur (NA/S), zinc/bromine (Zn/Br), Vanadium Redox Flow (VRF) and others.
These Electricity Storage Systems can also be categorized into High energy capacity (e.g. VRF, Na/S) and High power rating (flywheel, lead acid, Li-Ion). High energy capacity storage system is designed to provide a power over long period of time e.g hours/day. Whereas high power rating storage systems are designed to provide higher power over a short period of time e.g. minutes/hour.
Generally commercial available battery energy storage systems have a similar layout. Batteries are connected to a power conditioning unit (PCU) which converts a variable DC voltage of the battery to a three-phase AC voltage. The generated AC voltage is often different to the grid voltage so that a transformer is needed to transmit power into the grid. In parallel to the battery energy storage system, a monitoring of the battery may be performed through a battery management unit (BMU). BMU controls battery specific parameters like state of charge, dis/charging process or temperature of the battery, as well as the PCU and connection to the electrical grid where power is supplied or received.